Implementing bufferless Direct Memory Access (DMA) controllers using split transactions

ABSTRACT

According to one embodiment a method for implementing bufferless DMA controllers using split transaction functionality is presented. One embodiment of the method comprises, generating a write command from a disk controller directed to a destination unit, the write command including an identifier, generating a read command from the disk controller directed to a source unit, the read command including an identifier which matches the identifier in the write command, the source unit transmitting read data on a split transaction bus, the read data including the identifier of the read command, and receiving the read data at the destination unit via the split transaction bus if the identifier of the read data matches the identifier of the write command.

FIELD OF THE INVENTION

The present embodiments of the invention relate generally toinput/output (I/O) processors and, more specifically, relate to directmemory access (DMA) controllers.

BACKGROUND

Many storage, networking, and embedded applications require fastinput/output (I/O) throughput for optimal performance. I/O processorsallow servers, workstations, and storage subsystems to transfer datafaster, reduce communication bottlenecks, and improve overall systemperformance by offloading I/O processing functions from a host centralprocessing unit (CPU).

Typically, the CPU(s) in the I/O processors program direct memory access(DMA) controller(s) to move data between specified sources anddestinations, such as between local memory and host memory. Once the DMAcontroller is programmed, it will generate a read command to thesource's interface or controller. This controller or interface willgenerate the read command for the source, and once it obtains the readdata will place that data on the bus to the DMA controller. Typical DMAcontrollers include buffers to temporarily store data when the data ismoved between sources and destinations, such as between host and localmemories. The DMA controller will accept the read data and store it inthe DMA controller data buffers. At this time, the DMA controller willgenerate a write command to the destination's interface or controller.The destination interface or controller will accept this write command.Finally, the DMA controller provides the write data being stored in theDMA controller data buffers to the destination interface or controllerin order to be written to the destination.

The use of DMA controller data buffers can lead to increased arearequirements, increased power requirements, and added complexity to theI/O processor. The use of DMA controller data buffers also slows downperformance and increases costs for I/O processors.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given below and from the accompanying drawings of variousembodiments of the invention. The drawings, however, should not be takento limit the invention to the specific embodiments, but are forexplanation and understanding only.

FIG. 1 illustrates a block diagram of one embodiment of a computersystem;

FIG. 2 illustrates a flow diagram of one embodiment for implementingbufferless DMA controllers;

FIG. 3 illustrates a flow diagram of another embodiment for implementingbufferless DMA controllers;

FIG. 4 illustrates a flow diagram of another embodiment for implementingbufferless DMA controllers; and

FIG. 5 illustrates a flow diagram of another embodiment for implementingbufferless DMA controllers.

DETAILED DESCRIPTION

A method and apparatus to implement bufferless direct memory access(DMA) controllers using split transactions are described. Reference inthe specification to “one embodiment” or “an embodiment” means that aparticular feature, structure, or characteristic described in connectionwith the embodiment is included in at least one embodiment of theinvention. The appearances of the phrase “in one embodiment” in variousplaces in the specification are not necessarily all referring to thesame embodiment.

In the following description, numerous details are set forth. It will beapparent, however, to one skilled in the art, that the embodiments ofthe invention may be practiced without these specific details. In otherinstances, well-known structures and devices are shown in block diagramform, rather than in detail, in order to avoid obscuring the presentinvention.

FIG. 1 is a block diagram of one embodiment of a computer system 100 toimplement bufferless DMA controllers using split transactions. Thesystem 100 includes a local memory 110, an I/O processor 120, anexternal bus 132, a host system 140, and a host memory 145. Embodimentsof the invention are not limited to being implemented with local andhost memories, but may generally be implemented between any source anddestination units accessed by an I/O processor.

The I/O processor 120 further includes a memory controller 122, a splittransaction bus 124, a CPU 126, a DMA controller 128, and an externalbus interface 130. Although embodiments of the invention reference theuse of DMA controllers, alternatively other disk controllers may beused. The I/O processor 120 provides for intelligent I/O with the helpof the CPU 126 and memory controller 122 coupled to the splittransaction bus 124. In one embodiment, CPU 126 is a processor in thePentium® family of processors including the Pentium® II processorfamily, Pentium® III processors, and Pentium® IV processors availablefrom Intel Corporation of Santa Clara, Calif. Alternatively, other CPUsmay be used.

The memory controller interfaces the local memory 110, and that localmemory 110 may include random access memory (RAM) such as SynchronousDynamic RAM (SDRAM). The local memory 154 includes the instructions anddata for execution and use by the CPU 126.

The split transaction bus 124 is a bus that is capable of supportingexplicit split transactions of both read and write commands. In oneembodiment, split transaction bus 124 is a XSI on-chip-bus.Alternatively, other split-transaction-capable buses may be used.

Generally, split transactions split the address and data informationphases of a data transfer. Splitting these phases is implemented byusing an identifier, for example a Sequence ID. The address and dataphases of the split transaction are coupled using the identifier. Duringthe address phase, the requesting unit provides an identifier withcommand and attributes.

During the data transfer phase, the unit supplying data uses the sameidentifier in order to tie the commands together at each unit. Duringreads, the agent which claimed the read command will supply data, whileduring writes the agent that generated the write command provides data.Additionally, a byte count may be used, along with the identifier, tocouple the address and data phases of a split transaction.

In one embodiment, the DMA controller 128 moves data from local memory110 to host memory 145, or alternatively, from host memory 145 to localmemory 110. The data transfer may take place without the use of databuffers in the DMA controller.

For example, to perform a data transfer from the host memory 145 to thelocal memory 110, the CPU 126 first programs the DMA controller 128 toperform the data transfer. The DMA controller 128 will then generate awrite command to the memory controller 122. This write command includesan Identifier (ID) and, in some embodiments, a Byte Count. The DMAcontroller 128 then generates a read command to the external interface130 with the same ID and Byte Count information.

The external interface 130 will then claim the read command and generatethe read command on the external bus 132. Once the external interface130 receives the read data from host system 140, it places the readdata, with the corresponding ID and Byte Count, on the split transactionbus 124. Finally, memory controller 122 will accept the read databecause the ID and Byte Count of the read data match the ID and ByteCount of the write command the memory controller 122 previously receivedfrom the DMA controller 128.

In another embodiment, to perform a data transfer from the local memory110 to host memory 145, the CPU 126 first programs the DMA controller128 to perform the data transfer. The DMA controller 128 will thengenerate a write command to the external bus interface 130. This writecommand includes an Identifier (ID) and, in some embodiments, a ByteCount. The DMA controller 128 then generates a read command to thememory controller 122 with the same ID and Byte Count information.

The memory controller 122 will then claim the read command and place theread data from local memory 110, with corresponding ID and Byte Count,on to the split transaction bus 124. Finally, external bus interface 130accepts the read data on the split transaction bus 124 because the IDand Byte Count match the ID and Byte Count of the write command theexternal bus interface 130 previously received from the DMA controller128.

In another embodiment, to perform a data transfer from one local memorylocation 110 to another local memory location 110, the CPU 126 firstprograms the DMA controller 128 to perform the data transfer. The DMAcontroller 128 will then generate a write command to the memorycontroller 122. This write command includes an Identifier (ID) and, insome embodiments, a Byte Count. The DMA controller 128 then generates aread command to the memory controller 122 with the same ID and ByteCount information.

The memory controller 122 will then claim the read command and place theread data from local memory 110, with corresponding ID and Byte Count,on to the split transaction bus 124. Finally, the memory controller 122accepts the read data on the split transaction bus 124 because the IDand Byte Count match the ID and Byte Count of the write command thememory controller 122 previously received from the DMA controller 128.

In another embodiment, to perform a data transfer from one host memorylocation 145 to another host memory location 145, the CPU 126 firstprograms the DMA controller 128 to perform the data transfer. The DMAcontroller 128 will then generate a write command to the external businterface 130. This write command includes an Identifier (ID) and, insome embodiments, a Byte Count. The DMA controller 128 then generates aread command to the external bus interface 130 with the same ID and ByteCount information.

The external interface 130 will then claim the read command and generatethe read command on the external bus 132. Once the external interface130 receives the read data from host system 140, it places the readdata, with the corresponding ID and Byte Count, on the split transactionbus 124. Finally, external bus interface 130 accepts the read data onthe split transaction bus 124 because the ID and Byte Count match the IDand Byte Count of the write command the external bus interface 130previously received from the DMA controller 128.

FIG. 2 depicts a flow diagram of one embodiment of implementingbufferless DMA controllers using split transactions. More specifically,the flow diagram depicts a data transfer from a host memory to a localmemory, under one embodiment of the invention. At processing block 210,the CPU programs the DMA controller to perform a data transfer. Usingthe split transaction functionality of the bus the DMA controllergenerates a write command to the memory controller with a unique ID anda Byte Count at processing block 220.

At processing block 230, the DMA controller generates a read command tothe external interface (using the source address) with the same ID andByte Count as the write command that was previously given to the memorycontroller. The external interface claims the read command at processingblock 240 and generates the read command on the external bus atprocessing block 250. Once the external interface receives the read datafrom the host system at processing block 260, it places this data on thesplit transaction bus at processing block 270. Finally, at processingblock 280, the memory controller accepts the read data found on thesplit transaction bus that matches the ID and Byte Count of the writecommand earlier given to it by the DMA controller.

FIG. 3 depicts a flow diagram of one embodiment of implementingbufferless DMA controllers using split transactions. More specifically,the flow diagram depicts a data transfer from a local memory to a hostmemory, under one embodiment of the invention. At processing block 310,the CPU programs the DMA controller to perform a data transfer. The DMAthen generates a write command (using the destination address) to theexternal interface with a unique ID and Byte Count at processing block320.

The DMA controller generates a read command (using the source address)to the memory controller with the same ID and Byte Count found in thewrite command at processing block 330. At processing block 340, thememory controller claims the read command, and, at processing block 350,returns the read data onto the split transaction bus. At processingblock 360, the external interface accepts the read data found on thesplit transaction bus that matches the ID and Byte Count of the writecommand it accepted previously.

FIG. 4 depicts a flow diagram of one embodiment of implementingbufferless DMA controllers using split transactions. More specifically,the flow diagram depicts a data transfer from one location in localmemory to another location in local memory, under one embodiment of theinvention. At processing block 410, the CPU programs the DMA controllerto perform a data transfer. Using the split transaction functionality ofthe bus, the DMA controller generates a write command to the memorycontroller with a unique ID and a Byte Count at processing block 420.

The DMA controller then generates a read command to the memorycontroller with the same ID and Byte Count found in the write command atprocessing block 430. At processing block 440, the memory controllerclaims the read command, and, at processing block 450, returns the readdata onto the split transaction bus. At processing block 460, the memorycontroller accepts the read data found on the split transaction bus thatmatches the ID and Byte Count of the write command it acceptedpreviously.

FIG. 5 depicts a flow diagram of one embodiment of implementingbufferless DMA controllers using split transactions. More specifically,the flow diagram depicts a data transfer from one location in hostmemory to another location in host memory, under one embodiment of theinvention. At processing block 510, the CPU programs the DMA controllerto perform a data transfer. Using the split transaction functionality ofthe bus, the DMA controller generates a write command to the externalinterface with a unique ID and a Byte Count at processing block 520.

At processing block 530, the DMA controller generates a read command tothe external interface with the same ID and Byte Count as the writecommand that was previously given to the external interface. Theexternal interface claims the read command at processing block 540 andgenerates the read command on the external bus at processing block 550.Once the external interface receives the read data from the host systemat processing block 560, it places this data on the split transactionbus at processing block 570. Finally, at processing block 580, theexternal interface accepts the read data found on the split transactionbus that matches the ID and Byte Count of the write command earliergiven to it by the DMA controller.

Although FIGS. 2 through 5 present implementing bufferless DMAcontrollers using split transactions in the context of data transfersbetween and within host and local memories, other embodiments may beimplemented, such as transferring data between a peripheral and the hostmemory. Generally the apparatus and methods presented can be implementedbetween any source and destination units between which an I/O processortransfers data.

Embodiments of the invention use split transaction functionalityprovided by split-transaction-capable buses to implement DMA controllerswithout data buffers. Removing data buffers from the DMA controller mayresult in increased performance and lower costs, as data is moveddirectly from one memory to the other memory, instead of beingintermediately stored in the DMA data buffer.

Whereas many alterations and modifications of the present invention willno doubt become apparent to a person of ordinary skill in the art afterhaving read the foregoing description, it is to be understood that anyparticular embodiment shown and described by way of illustration is inno way intended to be considered limiting. Therefore, references todetails of various embodiments are not intended to limit the scope ofthe claims, which in themselves recite only those features regarded asthe invention.

1. An apparatus, comprising: an input/output processor comprising: adisk controller to generate a write command and to generate a readcommand directed to a source unit, the write command including a writecommand identifier and the read command including a read commandidentifier that matches the write command identifier, wherein the writecommand further includes a byte count, and the read command furtherincludes a byte count that matches the byte count of the write command;a split-transaction bus to transfer read data, the read data transmittedon the bus by the source unit in response to the read command andincluding the read command identifier; and a destination unit to receivethe write command and to remove the read data from the split transactionbus if the write command identifier and the read command identifiermatch and if the byte count of the read data matches the byte count ofthe write command.
 2. The apparatus of claim 1, wherein the destinationunit is local memory, and the source unit is host memory.
 3. Theapparatus of claim 1, wherein the destination unit is host memory, andthe source unit is local memory.
 4. The apparatus of claim 1, whereinthe destination unit is one location in local memory, and the sourceunit is another location in local memory.
 5. The apparatus of claim 1,wherein the destination unit is one location in host memory, and thesource unit is another location in host memory.
 6. A system, comprising:a local memory; and an input/output processor having: a disk controllerto generate a write command and to generate a read command directed to asource unit, the write command including a write command identifier andthe read command including a read command identifier that matches thewrite command identifier, wherein the write command further includes abyte count, and the read command further includes a byte count thatmatches the byte count of the write command; a split-transaction bus totransfer read data, the read data transmitted on the bus by the sourceunit in response to the read command and including the read commandidentifier; and a destination unit to receive the write command and toremove the read data from the split transaction bus if the write commandidentifier and the read command identifier match and if the byte countof the read data matches the byte count of the write command.
 7. Thesystem of claim 6, wherein the DMA controller does not include buffers.8. The system of claim 6, wherein the input/output processor furthercomprises: a central processing unit connected to the split transactionbus; a memory controller connected to the split transaction bus; and anexternal bus interface connected to the split transaction bus.